1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a master slice semiconductor device.
2. Description of the Prior Art
Increasing use has been made of master slice semiconductor devices in recent years so as to improve customer service. In a master slice semiconductor device (hereinafter referred to as "master slice IC"), many basic circuit elements are formed in advance, and are thereafter suitably connected by conductors to form a variety of IC products. In other words, a variety of circuit functions needed by individual users can be obtained from the same bulk circuit elements by suitably arranging conductors.
The most popular type of master slice IC uses polysilicon gates. This type of master slice IC is composed of metal oxide semiconductor (MOS) transistors fabricated by polysilicon as their gate electrodes and is advantageous for miniaturizing IC patterns and, therefore, realizing a high density.
While polysilicon gate type master slice IC's are the most popular, at present, another type of master slice IC, which uses metal gates, is still in strong demand, for example, in the field of small scale integration (SSI) devices or middle scale integration (MSI) devices. This type of IC offers some advantages over a polysilicon gate type, such as a higher withstand voltage and lower manufacturing cost.
The high withstand voltage is due to the usual 10 .mu.m width, i.e., channel length, of a metal gate, as compared with the usual 1 .mu.m width of a polysilicon gate.
The low manufacturing cost is due to the lower number of masks used for manufacturing a metal gate IC than for a polysilicon gate IC.
Metal gate IC's, however, are not suited for highly dense IC's or simple pattern designs due to the wide width of the metal gates and to the need for complicated arrangement of the power lines and signal lines on the IC substrate.
The metal gates could be made narrower. While this would cause at most no technical trouble, it would entail a costly overhaul of existing metal gate IC manufacturing lines.
As for the complicated arrangement of lines, this is an inherent disadvantage of metal gate IC's. Polysilicon gates can freely intersect with power lines, signal lines, and other metal conductors since each polysilicon gate is usually covered by an insulation film and, thus, no short circuits are expected to occur. Metal gates, however, must be kept from intersecting with metal conductors, since the metal is not covered by an insulation film.
In such a case, so-called diffused resistor regions are very useful so as to form multilevel crossings where such intersections are inevitable. In a master slice IC, however, not all the diffused resistor regions will finally be used, thus resulting in needless space occupied by unused regions. This space must be compensated for to obtain high IC integration.